<?php
/*限制文件上传类型 https://www.mageplaza.com/devdocs/how-file-upload-sytem-configuration-magento-2.html*/
namespace Mageplaza\HelloWorld\Model\Config\Backend;

class CustomFileType extends \Magento\Config\Model\Config\Backend\File
{
    /**
     * @return string[]
     */
    public function getAllowedExtensions() {

        return ['csv', 'xls','jpg'];
    }

}